The composition of the vapour
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص179-180
2025-11-15
32
The composition of the vapour
The compositions of the liquid and vapour that are in mutual equilibrium are not necessarily the same. Common sense suggests that the vapour should be richer in the more volatile component. This expectation can be confirmed as follows. The partial pressures of the components are given by eqn 6.2. It follows from Dalton’s law that the mole fractions in the gas, yA and yB, are
Provided the mixture is ideal, the partial pressures and the total pressure may be ex pressed in terms of the mole fractions in the liquid by using eqn 6.2 for pJ and eqn 6.3 for the total vapour pressure p, which gives
Figure 6.7 shows the composition of the vapour plotted against the composition of the liquid for various values of p* A/p* B> 1. We see that in all cases yA> xA, that is, the vapour is richer than the liquid in the more volatile component. Note that if B is non-volatile, so that p* B = 0 at the temperature of interest, then it makes no contribution to the vapour (yB= 0).
Equation 6.3 shows how the total vapour pressure of the mixture varies with the composition of the liquid. Because we can relate the composition of the liquid to the composition of the vapour through eqn 6.5, we can now also relate the total vapour pressure to the composition of the vapour:
Fig. 6.7 The mole fraction of A in the vapour of a binary ideal solution expressed in terms of its mole fraction in the liquid, calculated using eqn 6.5 for various values of p*A/p* B (the label on each curve) with A more volatile than B. In all cases the vapour is richer than the liquid in A.
This expression is plotted in Fig. 6.8.
Fig. 6.8 The dependence of the vapour pressure of the same system as in Fig. 6.7, but expressed in terms of the mole fraction of A in the vapour by using eqn 6.6. Individual curves are labelled with the value of p*A/p* B.
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